Vitrified grinding wheel



June 13, 1944.

G. N. JEPPSON ET AL VITRIFIED GRINDING WHEEL 2 Sheets-Sheet 1 Filed May 5, 1942 F 4 550E755 N JEPF'EUN Mm 7'0N FEEECHEH June 13, 1944. N EPP ON ET AL 2,351,433

VITRIFIED GRINDING WHEEL FilecLMay 5, 1942 2 Sheets-Sheet 2 Qwuwvfmu EEQREE NJEF'PSDN M/L. Ta/v FBEECHER Patented June 13, 1944 I VITRIFIED GRINDING WHEEL George N. Jeppson, Brookfield, and Milton F. Beecher, Holden, Mass, assignors to Norton Company, Worcester, Mass, a corporation of Massachusetts ,Application May 5, 1942, Serial No. 441,804

Claims. (01. 51-206) The invention relates to vitrified grinding wheels.

- One object of the invention is to provide a vitrified grinding wheel which will give a performance in snagging operations comparable to that of a resinoid bonded grinding wheel. Another object of the invention is to provide a vitrified grinding wheel which is extremely resistant to all kinds of stresses customarily met with in snagging operations, such as centrifugal force and thermal shock.

Another object of the invention is to extend the field of usefulness of vitrified bonded grinding wheels so that, for example, they can be economically used when certain'organic materials are not obtainable. Another object of the invention is to provide a vitrified grinding wheel capable of being run at higher speeds than have been customary for vitrified grinding wheels and capable of emcient use in snagging operations *at such high speeds. Other objects will be in part obvious or in part pointed out hereinafter. The invention accordingly consists in the features of construction, combinations of elements. arrangements of parts, and'in the several steps and relation and order of each of said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings illustrating one of many possible embodiments of the-mechanical features of this invention, v

Figure 1 is an axial sectional view of a mold filled with dry granular mixes for the manufacture of a grinding wheel.

Figure 2 is a view similar to Figure 1 showing the first top plate in position.

Figure 3 is a view similar to Figure 2 showing the lugs after they have been driven down.

Figure 4 is a view similar to Figure 3 after the second and third top plates have been placed in position.

Figure 5 is a plan view of the mold in the condition shown-in Figure 3.

Figure 6 is a plan view of of the mold after the second top plate has been placed in position but before the third top plate has been inserted.

Figure '7 is an axial sectional view showing the mold after it has been closed.

Figure 8 is an elevation of a grinding wheel made in accordance with the invention, and

We provide a quantity of abrasive material, such as electric furnace fused alumina, corundum, emery, or silicon carbide, in a plurality of grit sizes. We also provide a quantity of ceramic bond. The bond may be clay, ceramic flux or ceramic frit or any combination of them. Certain clays, such as the ball clays, are plastic; if glasses or frits alone are used, a temporary binder in the nature of dextrin orother organic Than I Bond analysis Oxide It is desired that the wheel have high resistance to breakage from centrifugal force which is ordinarily measured by the breaking speed of the wheel in surface feet per minute. It happens to be a general law of disks and cylinders that a given composition of matter embodied therein will break at approximately the same speed expressed in surface feet per minute regardless of the size of the wheel. Whether this law holds good for all sizes is immaterial, since it holds good for the ranges of sizes involved in the manufacture of grinding wheels. So, therefore, the breaking speed in surface feet per minute (not R. P. M.) is a proper measure of the resistance to breakage of the material of which the wheel is formed. k

It has been demonstrated that a homogeneous grinding wheel usually starts to break at the central hole. The stresses due to centrifugal force are concentrated at the center of the wheel. Instead of making the entire wheel of very strong material, we find it advantageous to make a composite wheel the outer portion .of which has the desired grinding characteristics, the inner portion being stronger and denser, thereby sacrificing the grinding characteristics of the inner portion. This is not detrimental since the strong inner portion is the discarding stub of the wheel which could not be used in grinding because of being too small for the required work.

Wheels run at high speeds heat up more than wheels run at low speeds, grinding pressures being assumed to be equal. Organic bonded wheels have a certain amount of flexibility or resiliency which vitrified wheels have to a considerably lesser degree. Consequently vitrified wheels are subject to breakage from thermal shock or difference in temperature between the outer portion and the inner portion. Since the heat is generated at the periphery where the grind-.

ing is done, a heat differential is set up in any grinding wheel under grinding conditions. Consequently despite the structure which permits higher speeds so far as centrifugal force is concerned, the vitrified wheel with such structure would be subject to breakage from thermal shock or heat differential. Accordingly we provide means to increase the resistance to heat differential or thermal shock. We carry this out by providing or forming in the wheel radial slots. These radial slots permit expansion of the outer zone of the wheel without setting up such strains as will crack or fracture the wheel. Preferably two rows of slots are provided to avoid making any one set of slots too long in a radial direction yet to provide some slots after the area of the first set has been worn away through grinding.

It is known that slotting a grinding wheel reduces its resistance to breakage due to centrifugal force, that is to say, a slotted wheel ought to break at a lower speed, theoretically. A surprising result of the invention is that the breaking speed of wheels is lowered only to a small extent by the slots which comprise the means for increasing the resistance to thermal shock, hereinafter called the heat factor. This discover makes possible the manufacture of wheels according to the invention which can be run at higher speeds successfully. The breaking speed is materially raised by the composite structure without sacriflce of grinding characteristics. The heat factor is substantially raised by the slottin without much loss of the gain in breaking speed. Thereby the wheels of the invention can be run at speeds now commonly used only with reslnoid or rubber wheels. There are certain grinding characteristics of vitrified wheels which are preferred and furthermore owing to the scarcity of rubber and reslnoid materials and the unlimited supplies of clay and the like, it is desired to. extend the use of vitrified wheels.

As an illustrative example of the present invention, the steps in the manufacture of a vitrifled grinding wheel will now be explained. Referring first to Figure 1, we provide a mixture III which is to form the outer or grinding part of the wheel, and a mixture I l which is to form the central part of the grinding wheel. We provide a mold comprising a mold band l2. a bottom plate I I and a central arbor I4. We further provide a thin cylindrical metal separator I5 which is illustrated in Figures 1 and 9. We charge the mold of Figure 1 with the mixtures I and II and level them off. We then withdraw the separator l. As shown in Figure 9, the bottom of the separator I! has saw toothed portions I6 formed by upsetting the metal outwardly. The withdrawal of the separator disturbs the mixtures and forms a rough rather than a smooth division between them, thus making a closer knit structure.

Referring now to Figure 2, we now place a mold plate II upon the two mixtures I0 and II, This mold plate l'l fits over the arbor I4 as shown and, as shown in Figure 5, has a series of radial slots I8 which extend to the periphery of the plate and another series of radial slots I9 which are spaced inwardly from the periphery. The outer circle of the slots I8 is farther from the center of the plate than the inner circle of the slots I8.

We now insert lugs 20 into the slots l8 and I9, one lug in each slot. These are rectangular parailelepipeds of steel, preferably hardened, having holes 2| for the insertion of an instrument for withdrawal; preferably the bottoms 22 of the lugs 20 are rounded.

Referring now to Figure 3, with a hammer and pin we drive downwardly each of the lugs 20. They can be driven down almost to the bottom of the mold and the material I0 is forced to one side in so doing.

Referring now to Figures 4 and 6, we provide an intermediate plate 23 having slots 2| and 25 spaced the same as the slots I8 and I9 but these slots 24 and 25 are slightly longer and slightly wider than the slots I8 and I9. We place this intermediate or second top plate 23 upon the first top plate I! and then place a plain third top plate 26, having no slots, upon the intermediate or second top plate 23, all as shown in Figure 4.

The mold is now ready for the hydraulic press. We provide the usual hydraulic press having platens 21 and 28 one of which is movable. The press is then operated to press the platens togather, thus closing the mold," that is to say preferably driving inwardly each of the bottom plate I3 and the set of top plates I1, 23 and 26. The mold is shown closed in Figure 7. The mold is stripped in the usual way, that is to say the mold band I2 and the arbor I are first removed, after which the third top plate 26 and the second top plate 23 can be readily removed and the lugs 20 can then be removed by levers inserted in the holes 2|. After that the first top plate I! can readily be slid oil the pressed wheel. The wheel is then fired in a kiln to vitrify the bond. As shown in Figure 7 the lugs 20 do not quite contact the bottom plate I3 and therefore portions 30 remain, but these portions disappear in the siding operation which shaves down one side of the wheel reducing the thickness thereof. The other side of the wheel and also the periphery may be sided to true it and the wheel is then complete and assumes the form shown in Figure 8.

Referring to Figure 8, the central zone is the dense, strong non-grinding zone II a, which merges with the outer grinding zone Ina at an irregular but generally circular line 3|. Slots l8a extend from the periphery inward and a second set of slots l9a are located in a circle just inside the slots I la. Before the slots Illa have disappeared due to wearing away of the wheel, the slots I9a will have been uncovered.

For the manufacture of the central portion Ila of the wheel we may provide a mixture of bond and abrasive of small grit size. For the manufacture of the outer zone Illa we preferably provide a mixture of bond and abrasive of larger grit size. In this manner the strength of the inner zone Ila is greater than that of the outer zone Illa. It is also preferred that the porosity of the inner zone should be less than that of the outer zone, for example to the extent of one or two per cent of porosity. It is likewise preferred that the inner zone be a little harder than the outer ascncss zone, that is to say having a slightly greater volume percentage of bond, for example an additional three per cent. In all of the following examples when the wheels had two zones, the grid size of the outer or grinding zone was I, while the grit size of the central zone was 48. We refer to the two zone wheels as composite wheels.

Every wheel the size of which is defined by four:

tral hole by 13 inches diameter of strong zone I la.

Hereinafter the figures 24 x 2 x 5 x 13 wil1represent such a wheel and other sized wheels will be similarly described, the first number representing the diameter of the wheel, the second number the thickness of the wheel, the third number the diameter of the centralhole, and the fourth number the diameter 01' the stronger inner zone, all stated in inches. This wheel was speed tested until it broke at 19,900 S. 1 P. M. (surface feet per minute). slots only, nine slots each two inches deep. An identical wheel with an outer row of slots was broken in a heat coil test and showed a heat factor of 69.

Two more wheels were made to exactly the same specifications excepting that they were not slotted. One of these broke in the speed test at 20,950 S. F. P. M. and the other was broken in the heat coil test and showed a heat factor of 48.4.

The heat factor test gives the resistance to breakage from heat differential or heat shock and is' determined by measuring the temperature near the periphery and the temperature near the center of the wheel while the wheel is being heated with a coil of wire wound around it through which passes an electric current. The higher the heat factor the stronger is the resistance of the wheel to breakage. The numbers are obtained according to the following formula. If

R =radius of wheel in inches R,,=radius of central hole in inches Y =temperature rise in degrees centigrade at position No. 1 near periphery X ,='distance of position No. 1 from center of wheel.

Y =temperature rise in degrees centigrade at position No. 2 near center X =distance of position No. 2 from center 2L Tf= A o) Another set of wheels was made up 24 x 2 x 5 x 13, several having an outer row of slots, nine in number, two inches deep, and several not being slotted. These differed from the first lot in being slightly less porous and having slightly greater volume percentage of bond in both portions. These wheels were identified by the number 031. One of the slotted wheels broke at a speed of 20,000 S. F. P. M.-and another slotted wheel had a heat factor of 51. One of the unslotted wheels This wheel had an outer row of broke at 20,500 8. 1'. P. M. and another unslotted wheel had a heat factor of 36.5.

Another comparison was made with wheels 24 x 2 x 5 x 13 with the number 032. A number were made up with slots, nine in number. two

'inches deep, and a number without any slots.

One or those with slots broke at a speed of 19,850 S. F. P. M. and another similar wheel with an outer row of slots, nine in number, two inches deep, had a heat factor of 56. One of the unslotted wheels was broken in the speed test at 21,350 S. F. P. M. andv the heat coil test gave a heat factor of 43.7. These wheels had slightly more bond and slightly less pores than the wheels 031.

With regard to all three lots of wheels, each of them had 54 volume per cent of abrasive in the outer or grinding portion Illa and 52 volume per cent of abrasive in the inner strong portion Ila.

In another comparison wheels were made up,

according to a formula identified by the number 441. These wheels were 24% x 2 x 2which means they were not of composite structure, that is to say they were of uniform abrasive structure from central hole to periphery. The first lot had no slots, the second lot had 25 slots an inch deep from the periphery in, and the third lot had 19 slots two inches deep from the periphery in. The breaking speed in surface feet per minute. heat factor, and the depths of the slots are shown by the following table. a

. Team: II

Depth of slots 5325? Heat factor Inches TAsLs: III

Depth of slots Heat factor Inches In a vitrified wheel a 2 inch central hole is typical. It is known that the smaller the central hole the stronger is the wheel, other things being equal. In resinoid snagging wheels 4 and 6" a central holes are typical. The resinoid being that of the outer zone of 031; and the structure and volume percentage of 035 was the same as that of the duter zone of 032. It will be remembered that the breaking speed of 030 was 19,900 8. F. P. M., that the breaking speed of 031 was 20,000 S. F. P. M. and that the breaking speed of 032 was 19,850 8. F. P. M. The breaking speed of 033 was 13,650 S. F. P. M., the breaking speed of 034 was 14,150 S. F. P. M. and the breaking speed of 035 was 13,800 S. F. P. M. Thus the breaking speed of typical-vitrified wheels (033, 034 and 035 were typical vitrified wheels) was materiallyeraised by the use of the composite structure despite the fact that in order that such wheels might be used where resinoid wheels are now used, they were made with central holes two and one half times as large, and further despite the fact that in order that the heat factor might be raised, they were slotted. Additional units of wheels identified by the numbers 033, 034, 035 were made up and broken in the heat coil tests. These gave heat factors for 03332.5; UMP-34.7: 035-315, comparing with the respective figures 69, 51 and 56 for similar wheels which were slotted and composite.

Immany cases it will be preferred to have only an outer row of slots. The inner row of slots actually makes the wheel weaker both in a speed test and heating test but it is useful because the wheel will have more heat resistance when the outer row of slots has disappeared. Nevertheless if the wheel is used on the same machine it will necessarily operate at a lower S. F. P. M. when it has a smaller diameter and this in itself may be enough compensation and therefore in most cases the inner row of slots may be omitted. Evidence on this point is given by a comparison of wheels known as 231 and 232. Each of these wheels was 20 x 2 1: 6 x 11. They were fairly hard both in the inner and outer portions, the portion Ila beingharder. They were identical in grain and volume percentage of abrasive, bond and pores. Wheel 231, with an outer row of slots only, seven slots, two inches deep, broke in the speed test at 17,800 S. F. P. M. while wheel 232 with two rows of slots broke in the speed test at 16,600 S. F. P. M. Wheel 232 had seven outer slots two inches deep, and seven inner slots one and one-half inches deep. Wheel No. 231 had a heat factor of 55 while wheel No. 232 had a heat factor of 46.

On the other hand, thevalue of the inner row of slots for some conditions is shown by the following:

Wheels 233 and 388 were composite wheels 16 1: 2 x 6 x 11. They had the same grit size of abrasive grain and the same volume percentage of abrasive, bond and pores in each of the twozones, respectively. Wheel 388 originally had a double row of slots (same specifications as wheel 232) and was originally larger so that a only the inner row was left. Wheel 233 had originally only an outer row of seven slots two inches deep, was originally larger and was worn away so that no slots were left. The heat factor of wheel 233 was 52 while that of wheel 388 was 7'1. The depth of the remaining slots in wheel 388 was 1% inches extending inwardly from the periphery.

Another comparison can be made between wheels 231 and 232 after they had been worn down and thereby reduced in size; these wheels previously identified were now 16 1: 2 x 6 x 11. Since wheel 232 had two rows of slots there was only the inner row left. Since wheel 231 had only an outer row of slots there were no slots left. Wheel 231 had a heat factor of 36 while wheel 232 had a heat factor of 54.

Another comparison that can bemade is between wheels 387, 262 and 388. All of these wheels were 20 x 2 x 6 x 11. All of these-wheels had the same volume percentage of abrasive,

bond and pores, respectively, in their inner and: outer zones. The differences between the wheels were solely in the slots; 387 had an outer row of seven slots two inches deep; both of 262 and 388 had two rows of slots, seven outer slots two inches deep, and seven inner slots one and onehalf inches deep. Wheel No. 387 broke in the speed test at 17,500 5. F. P. M. and gave a heat factor of 71. Wheel No. 262 broke in the speed test at 16,000 S. F. P. NL; wheel No. 388 had a neat factor of 53.1 It is to be remarked that these speed tests showed high breaking speeds even though the central hole is 6 inches in diameter whereas in the typical vitrified wheels the central holes are only 2 inches in diameter and they break at much lower speeds. It is also to be observed that in the two wheels 387 and 388 which were given the heat test, the heat factor is well above that found for unslotted wheels.

In the foregoing various wheels are identified by certain numbers. The figures may show that a wheel of a certain number broke at a certain speed in the speed test and gave a certain heat factor. The same wheel could not give both results because these tests destroy the wheels. It will, therefore, be understood that a wheel number represents a manufacturing check and two, three, four or half a dozenwheels may have been made on a given manufacturing check.

From the data which have been collected it is seen that vitrified wheels can be made with larger central holes to fit on the heavy spindles used for resinoid wheels in snagging operations, and can be run at speeds now considered safe only for resinoid wheels if they are made with a composite structure and are alsoslotted, in accordance with our invention. In certain cases an outer row of slots only will be employed and in other cases two rows of slots will be used. We have discovered that although slotting is supposed to weaken wheels, slotted and composite wheels even with larger central holes have far higher resistance to centrifugal force and to heat shock than wheels not so made and that it is important to embody both of these features in the wheels to permit them to be run on machines constructed for resinoid wheels at the high speeds at which resinoid wheels are now run. The added strength of the centre is obtained by a different structure, particularly a greater volume percentage of bond and concurrently a lower volume percentage of pores. If this ob- Jective were carried out without reducing the grit size of the abrasive, molding and shrinkage difliculties would be encountered. So, therefore, usually and characteristically wheels according to the present invention have inner portions with abrasive of smaller grit size than the outer portions.

One result of making the inner portion stronger while using the same kind of abrasive and bond .is that the inner portion will stretch more than the outer portion before it ruptures. This significantly increases the resistance of the wheel to heat shock. So, therefore, the composite structure as well as the slots increases the resistance to heat shock.

It will thus be seen that there has been provided by this invention an article and a method in which the various objects hereinabove set forth together with many thoroughly practisai advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limit ing sense.

We claim:

1. A vitrified grinding wheel having a relative- 1y large central hole and correspondingly a relatively low ratio of external radius to internal radius, said wheel being characterized by structural features for substantially compensating for diminished resistance, due to its relatively large internal radius, to the stresses of centrifugal force and for opposing the otherwise additive weakening effect due to heat differential, said features comprising an outer grinding zone of bonded abrasive grain and an inner zone of bonded abrasive grain of smaller grit size than said first-mentioned grain and of greater volume percentage of bond than that of said outer grinding zone, there being a series of slots extending inwardly by an appreciable distance from the periphery of said outer grinding zone for permitting expansion of said outer zone and thereby preventing the super-imposition of cracking strains upon said inner zone, said smaller grit size and greater volume percentage of bond of said inner zone giving the wheel suflicient strength to resist the stresses caused by centrifugal force and to compensate for the weakening efi'ect of said slots.

2. A vitrified snagging wheel having increased resistance of the wheel as a whole to heat shock comprising an outer peripheral zone slotted radially to permit thermal expansion of the outer.

zone without setting up strain therein which will fracture the wheel and an inner zone supporting the said outer zone, and made of the same abrasive and bond as the outer zone, but compacted to greater density than that of said outer zone.

3. A vitrified composite snagging wheel comprising inner and outer zones, formed of the same abrasive material and bond, in which the outer zone is structurally weakened but made more resistant to heat shock by slots extending through the wheel radially and the strength of the center zone is increased and the wheel as a whole made more resistant to heat shock by compacting said center zone to a greater density than that of said outer zone.

4. A vitrified grinding wheel having a relatively large central hole and correspondingly a relatively low ratio of external radius to internal radius, said wheel being characterized by structural features for substantially compensating for diminished resistance, due to its relatively large internal radius, to the stresses of centrifugal force and for opposing the otherwise additive weakening effect due to heat differential, said features comprising an outer grinding zone of bonded abrasive grain and an inner zone of bonded abrasive grain of smaller grit size than said first-mentioned grain and of lesser porosity than that of said outer grinding zone, there being a series of slots extending inwardly by an appreciable distance from the periphery of said outer grinding zone for permitting expansion of said outer zone and thereby preventing the superimposition of cracking strains upon said inner zone, said smaller grit size and lesser porosity of said inner zone giving the wheel sufilcient strength to resist the stresses caused by centrifugal force and to compensate for the weakening efiect of said slots.

5. A vitrified grinding wheel having a relatively large central hole and correspondingly a relatively low ratio of external radius to internal radius, said wheel being characterized by structural features for substantially compensating for diminished resistance, due to its relatively large internal radius, to the stresses of centrifugal force position of cracking strains upon said inner zone,

said lower porosity and greater volume percentage of bond or said inner zone giving the wheel suflicient strength to resist the stresses caused by centrifugal force and to compensate for the weakening eil'ect of said slots.

GEORGE N. JEPPSON.- MILTON F. BEECH. 

